1. Field of the Invention
The present invention is directed to a marker for employment in a electronic article surveillance system, the marker being of the type composed of one or more oblong, ductile magnetostrictive strips composed of amorphous ferromagnetic material that experience a change of the resonant frequency due to a change of the pre-magnetization field and that can be excited to longitudinal, mechanical resonant oscillations at the resonant frequency f.sub.r by a magnetic alternating field, whereby the mechanical stresses resulting from the resonant oscillations cause a change of the magnetization of the strips and, thus, a detectable modification of the alternating magnetic field.
2. Description of the Prior Art
When a strip of amorphous magnetostrictive material is exposed to a magnetic field, then the magnetostriction causes a change in length of the strip. Given positive magnetostriction, the strip will lengthen with increasing magnetic field. This dependency, however, is not linear but is dependent on the dimensions of the strip and on the size of the magnetic field. When the magnetic field is increased step-by-step by the respectively same amount at a specific strip, then it can be found that only small length changes initially ensue, that the length changes become greater with increasing steps for the increase of the magnetization, and that no further length change ensues given the occurrence of saturation despite continuing to increase the magnetic field step-by-step.
This property allows such a strip to be excited to mechanically oscillate when it is exposed to a pre-magnetization field whose size results in a great length change given the same modification of the magnetic field. Further, the length change ensuing due to the magnetic field causes the length of the strip to change in this area without having a tensile force act on the strip.
The modulus of elasticity of the material is the critical factor for the resonant frequency of the oscillation in the mechanical oscillation of a strip. The force required for a specific length change is all the greater and the resonant frequency of the oscillating strip becomes all the higher the higher this modulus of elasticity is. However, an additional length change ensues due to the influence of the magnetic field without requiring a force. The material thus acts as though it had a modulus of elasticity lower than the mechanical modulus of elasticity.
This results in the resonant frequency becoming lower than it is without pre-magnetization of the strip given increasing pre-magnetization with excitation of mechanical oscillations by a magnetic alternating field. A strip that oscillates with high signal amplitude at a given pre-magnetization with a specific resonant frequency will oscillate significantly less given excitation with the same frequency when the pre-magnetization field is removed, since the resonant frequency thereby increases and the exciting frequency and the resonant frequency no longer coincide.
Moreover, the effect of the removal of the pre-magnetization field is that a modification of the magnetic field only results in relatively slight modifications in the length of the strip, so that the signal height significantly decreases without pre-magnetization field.
Together, the two factors result therein that a mechanical oscillation of the strip is suppressed given removal of the pre-magnetization field. It is thus possible to deactivate a marker of this material in that the pre-magnetization field is removed.
This is achieved in markers, for example, having a semi-hard magnetic, further strip connected to the soft magnetic strip is demagnetized. Given other systems wherein the pre-magnetization field is co-generated by a coil in the examination region, the oscillation can be suppressed in that this pre-magnetization field is turned off.
PCT Application WO 88/01427 discloses a merchandise security system operating on a magneto-elastic basis which can detect not only whether one or more activated markers are present in the security area, but also, detect the number of activated display elements and/or their identity in the security area. PCT Application WO 88/01427 further discloses that the identification given the employment of magneto-elastic markers is essentially based on the presence of a number of resonant frequencies in the security area is interrogated. Since only the projection of the external field onto the longitudinal strip axis is effective in the display elements themselves, this makes it possible to detect the different strip orientations. For example, the individual objects can be individually identified and spatially resolved as well as a result on the basis of different strip orientations within a display element.
PCT Applications WO 90/03652 also discloses alloys having high magnetostriction that, in addition to containing iron, essentially contain nickel, and these are proposed for display elements of the species initially cited. These alloys, however, do not exhibit great changes of the resonant frequency with the pre-magnetized field and also do not simultaneously have a high signal amplitude and a good resonator quality. Markers that are provided with strips of such alloys exhibit only a short post-pulse oscillation of the signal after the deactivation of the external excitation, so that their detectability if greatly limited.
U.S. Pat. No. 5,495,231 also discloses markers for employment in a magnetic merchandise monitoring system that are composed of one or more oblong, ductile, magnetostrictive strips composed of amorphous ferromagnetic material that experience a resonant frequency change df.sub.r /dH.sub.Bias due to change of a pre-magnetization field H.sub.Bias, and which can be excited by an alternating magnetic field to mechanical resonant oscillations at the resonant frequency f.sub.r, whereby the mechanical stresses resulting from the resonant oscillations effect a change in the magnetization of the strip and, thus, a detectable modification of the alternating magnetic field. The alloys proposed therein exhibit a flat B-H loop that proceeds optimally linearly up into the region of saturation, whereby a strip of the material exhibits a magnetic anisotropy transverse to the longitudinal trip direction and the anisotropy field strength H.sub.aniso is higher than the pre-magnetization field strength H.sub.Bias.